Such heat exchange devices are used on a large scale in many branches of industry, e.g. in the petroleum industry for cooling products obtained from hydrocrackers and reactors for partial oxidation of (hydro)carbon-containing fuels such as natural gas, oil, coal and the like.
Typically the heat exchange takes place by passing the hot gasses through a pipe and contacting the exterior of the pipe with the coolant medium. If the coolant medium is water steam can advantageously be produced as a by-product of the heat exchange process in the apparatus.
When for cooling purposes the hot gases are passed through tubes which are cooled with a cooling medium on the outside, the walls of the tubes acquire a high temperature owing to transfer of heat from the hot gases to the tube metal which heat is further transmitted to the cooling medium.
An example of a hot gaseous medium to be cooled is the hot synthesis gas produced by partial oxidation of (hydro)carbon-containing fuel is generally cooled in a heat exchanger located next to the gasifier thereby producing high pressure steam. A critical area is the gas inlet of the heat exchanger where the hot synthesis gas enters the pipes of the heat exchanger apparatus. The wall thickness of the inlet area is to be minimised but should be thick enough to ensure mechanical integrity based on pressure and thermal loads. The gas velocity at the inlet area should be sufficiently high to prevent fouling (say 12 m/s) but on the other hand low enough to ensure sufficiently low gasside heat transfer coefficients. In particular, obtaining an optimum between fouling and velocity is desirable.
U.S. Pat. No. 3,610,329 describes a heat exchanger for cooling the hot gasses from a gasification process. The vertical oriented inlet section is protected against excessive heat by bricks located between the support plate and the space from where the hot gas flows. The tubes through which the hot gas flows transverse through the bricks. In an embodiment of U.S. Pat. No. 3,610,329 as shown in FIG. 4 the inlet section of the tubes is cooled by using a coolant having a low temperature. The used coolant is discharged from the inlet section by means of a separate conduit. A disadvantage of this design is the use of the bricks which in use can fall down.
An improved design, not having to make use of bricks, is described in U.S. Pat. No. 5,671,807 and U.S. Pat. No. 4,245,696. These publications describe an inlet section of such a heat exchange apparatus wherein the pipes are mounted at their upstream ends in an additional tube plate, also referred to in this description as a thermal shield, and a support plate. Between the thermal shield and the support plate a space is present to which coolant medium is added. This coolant medium will cool the thermal shield and the exterior of the pipes and is discharged from said space via an annular space surrounding the pipes when they pass the support plate. In said annular space the coolant medium flows co-current with the hot gas flowing in the pipes.
EP-A-290812 describes a heat exchanger vessel having an inlet section wherein the pipes are mounted at their upstream ends in a support plate. The coolant used to cool the inlet section is subsequently passed along the downstream end of the tubes via a second tube surrounding the entire length of the tubes. These jacket type of external tubes have openings to allow coolant to be discharged into the space surrounding the tubes and be discharged from the vessel via a single outlet opening.
EP-A-774103 describes an apparatus having an inlet section wherein the pipes are mounted at their upstream ends in a thermal shield and a support plate. Between the thermal shield and the support plate a space is present. As in the above references an annular space is present around the pipes as they pass the support plate. To said annular space coolant medium is added such that it passes the support plate in a direction counter-current to the hot gas flowing in the pipes. Additionally a skirt is present to guide the coolant medium along the exterior of the pipes into the space between thermal shield and support plate. The coolant is discharged at a position near the thermal shield and discharged via openings in the support plate to the main coolant compartment.
A disadvantage of the above prior art apparatuses is that the cooling of the upstream end of the pipes and the thermal shield is not sufficient. The present invention involves a design that aims to overcome said disadvantage.